DAOC - Dual Analag Optical Compressor
The kubi advanced version
These units are two of the very first prototypes of [silent:arts]'s take on of Analag's optical compressor design. I reckon they might be #3 or #4 of the units ever built that are based on [silent:arts]'s PCB. I was beta tester and builder for the PCBs before the batch production began.
It's not exactly the official version released later, but a modificated design instead, the so called "kubi advanced version" as named by [silent:arts] himself. I simply felt there was room for some inofficial improvements with the official PCB - and there definitely was!
Many of you guys have ordered the Analag opto compressor PCBs from Volker to start stuffing and building soon. Volker already pointed out in the help thread at ProPro that there is also a modified version by me which he called the kubi advanced version - I like the name.
Weeks before the PCBs were finished Volker and I had an email correspondence in which I pointed out my disfavor about some components and component values within the compressor and asked him to add some holes and features to the PCB to later allow the modifications - only if desired, of course.
Hope you like it! It took ages to convince Volker of the advantages to add the provisions for the modifications.
Add a second vactrol unit for a full wave detector. You might have wondered why there are ten holes for the vactrol even the vactrol needs only five, though. Insert the second vactrol reversed (one with the printing down to the PCB and one up). That will not only make the compressor more sensitive and give it more bite (and balls), it will also help to correctly and reliably handle asymmetrical waveforms like human voices - especially the male.
Increase Cx04 from 100nF to whatever you can maximally fit on the PCB. 220nF was the biggest cap I could insert. I wish there was enough space for 1uF, but real estate was too precious on the PCBs and Volker was too stingy with the real estate. Anyway, the diode within the vactrol is a variable resistor which forms a CR highpass filter with that cap. If you increase the cap, the filter frequency lowers and the compressor becomes more sensitive to lower frequencies. Yes, I know everybody wants a highpass switch for side chains like the GSSL's, but you don't need it here, 100nF is too small for many applications. Even worse: If you push the compressor harder, more current goes though the diodes and the resistance lowers so that less compression happens - quite inept, isn't it?
Don't worry, if you double the vactrols the resistance from two parallel diodes does not become lower, because only one diode conducts at a time, thus the other diode becomes an infinitive resistance for the period of a half signal wave (apart from the leakage current).
Change the input transformer from 10k:10k to 600:10k. Can be the same like the output transformer. 1:3 is cool, too.
600Ohms is high enough to be driven from any professional gear from the last 60 years. The changed transformer will give you additional 12dB gain at the input which you might really need as the compressor needs to be driven hard. It will also marginally improve the noise figures as a side effect due to the lower resistance.
Increase Cx02 to 1uF for more bottom end and a better phase linearity at lower frequencies. Instead of buying bigger caps, just swap Cx02 and Cx05. Cx05 can perfectly fulfill its purpose with a way lower capacity, too.
(For the record: Cx05 is meant to cancel out hum out of the output stage, kind of poor man's Aikido configuration. In fact that means 50Hz is the lowest frequency to pass. 1uF is giving less than 1Hz!)
PSU: Here we also have various component value changes from the original schematic and some more components like bypass caps got thrown in, but all these changes made it into the "official" version that everybody builds - no need to modify anything here.
Solen 10uF (Cx06) is a waste of money. Insert a decent 10uF - 100uF electrolytic and bypass it with a decent film cap. You will save heaps of money and will never hear a difference - it might even sound better.
I myself have substituted the 10uF Solen cap with 100uF JJ ANH + 1uF Solen + 15nF ERO KP1832. Sounds like an overkill to you? But it saved me money and sounds perfect to my ears.
The heavily increased coupling cap values will result in an frequency response flat from almost 1Hz on - measured without any transformers. Way too low? Yes, but that also shifts the phase shifts almost out of the audio band which was the main goal. The input and output transformers are now the components to determine your compressors bandwidth.
Enjoy! Please, feel free to build it, too, I highly recommend it. And please, let us know what you reckon after you modified your compressor!
PS: "marginally lowered noise" as mentioned a few times before means: It will theoretically lower noise, but you will not notice it.
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Darius Kubarth 2008